Despite numerous advances over the years, there still exists a continuing need for water purification. Many areas of the world have insufficient fresh water for drinking or agricultural uses, and in other areas where plentiful supplies of fresh water exist, the water is often polluted with chemical or biological contaminants, metal ions and the like. There is also a continuing need for commercial purification of other fluids such as industrial chemicals and food juices. U.S. Pat. No. 4,759,850, for example, discusses the use of reverse osmosis for removing alcohol's from hydrocarbons in the additional presence of ethers, and U.S. Pat. No. 4,959,237 discusses the use of reverse osmosis for orange juice.
Many of these needs have been addressed by filtration, and in particular by reverse osmosis, in which constituents are separated under pressure using a semi-permeable membrane. As used herein, the term membrane refers to a functional filtering unit, and may include one or more semi-permeable layers and one or more support layers. Depending on the fineness of the membrane employed, reverse osmosis can remove particles varying in size from the macro-molecular to the microscopic, and modern reverse osmosis units are capable of removing particles, bacteria, spores, viruses and even ions such as Cl.sup.- or Ca.sup.++.
There are several problems associated with large scale reverse osmosis (RO), including excessive fouling of the membranes and high costs associated with producing the required pressure across the membranes. These two problems are interrelated in that most or all of the known RO units require flushing of the membranes during operation with a relatively large amount of feed liquid relative to the amount of permeate produced. The ratio of flushing liquid reject to permeate recovery in sea water desalination, for example, is about 3:2. Because only some of the sea water being utilized is recovered as purified water, energy used to remaining water is wasted, creating an inherent inefficiency.
There have been numerous attempts over the years to improve the efficiency and concomitant cost effectiveness of RO units. U.S. Pat. No. 5,229,005 to Fok et al, for example, describes lowering a vessel from the side of boat deep into the ocean. The vessel is equipped with an RO membrane on one of its surfaces, and at a depth of about 700 meters, the pressure at depth is sufficient to force fresh water through the membrane and into the vessel. When the vessel is thus filled with fresh water, it is raised back to the ship and emptied. To increase operating efficiency, the inventor suggests alternately lowering and emptying two such vessels. While the claimed method can be functional, the non-continuous nature of the process renders it largely inadequate to supply fresh water on a commercial scale.
Another attempt at improving the cost effectiveness of RO units is discussed in U.S. Pat. No. 4,512,886 to Hicks et al. There, an RO module is placed in the ocean at a depth at which the ambient pressure is insufficient to operate the membrane, but at which the depth pressure combined with additional pressure provided by a pump is sufficient to operate the membrane. Pressurized water is therefore pumped through the RO module utilizing energy from waves overhead, with fresh water coming out one end of the module, and brine being eliminated from the other end. Unfortunately, the mechanism is limited to localities having considerable wave action, and in any event is relatively costly to install and operate.
Still another attempt at improving the cost effectiveness of RO units is discussed in U.S. Pat. No. 3,456,802 to Cole et al. In that patent, several RO cells are submerged at a sufficient depth in the ocean, and pre-filtered salt water is filtered at the surface and fed down to the cells through a pipe. Fresh water output of the cells is then pumped back up to the surface, while the flush water is returned to the ocean. By this mechanism Cole et al. claims to increase membrane life by pre-filtering the salt water applied against the membranes, and by increasing the flushing rate. What was not overcome, however, was the requirement of proximity to a deep body of salt water, and the difficulty in replacing the RO cells.
The requirement of proximity to a deep body of salt water in desalinization operations is addressed in U.S. Pat. No. 4,125,463 to Chenoweth, which is incorporated by reference herein in its entirety. In Chenoweth, numerous semi-permeable membrane assemblies are placed inside a well or other subterranean cavity. Salt water flows down to the membranes from above, and the hydrostatic pressure of the salt water drives a permeate through the membranes. The permeate, which in this case is purified water, is then pumped out of the system through a riser. The main advantage contemplated by Chenoweth is that energy expenditures are largely restricted to pumping the purified water.
Despite the reduced energy expenditures contemplated by Chenoweth, the design is impractical. Among other things, the Chenoweth design teaches a central riser surrounded at many different depths by clusters of five satellite RO units. Each of the satellite units has its own collector, and the various collectors of each cluster flow together at a manifold into the central riser. Such a design is inherently inefficient. Clustering of satellite RO units adds unnecessary complexity and expense, and the presence of multiple satellite casings on the same level wastes precious channel volume.
Thus, there still exists a need for apparatus and methods to cost effectively purify large quantities of fluid using pressurized filtration.